Two human diseases, Central Core Disease (CCD) and Malignant Hyperthermia (MH), are known to arise from mutations in the skeletal muscle Ca2+ release channel (ryanodine receptor, RYR1) of the sarcoplasmic reticulum (SR). These mutations cause the channel to be leaky to Ca 2+ and/or to open more readily in response to activators. The leak properties of this channel are regulated, at least in part, by the immunophilin, FKBP12. Our working hypotheses in this application are: a) FKBP12 binds to a noncontiguous binding site within the RYR1 tetramer to stabilize a closed state of the channel and allow cooperative opening of the channel, b) FKBP12.6 can substitute for FKBP12 in this role, c) the loss of FKBP12 modulation of RYR1 produces MH/CCD-like changes in muscle (Ca 2+ leak, enhanced sensitivity to caffeine, muscle atrophy, central cores, and/or fiber type switching) and d) MH/CCD mutations alter the interactions of RYR1 with FKBP12. To test these hypotheses we propose to: 1) determine if FKBP12 deficiency produces changes in skeletal muscle similar to changes associated with MH and CCD. 2) Analyze the interaction of FKBP12 and FKBP12.6 with RYR1. 3) Mutate the FKBP12 binding site on RYR1 and determine the effects on muscle structure and function. 4) Create mice with an MH/CCD mutation and analyze muscle structure and function. The first specific aim will use our newly created skeletal muscle specific FKBP12/FKBP12.6 mice, while specific aim 2 will be performed with rabbit skeletal muscle. Specific aims 3 and 4 will require the creation of two new strains of mice: one with a mutation in the FKBP12 binding site on RYR1 and one with an MH/CCD disease causing mutation.